Laser device

ABSTRACT

The laser device has a gain medium, first and second clads sandwiching the gain medium in the thickness direction, and a cavity structure for resonating the electromagnetic wave generated in the gain medium. The gain medium includes a plurality of active regions for generating an electromagnetic wave and at lease one connecting region sandwiched among the active regions. The first and second clads are each formed of a negative permittivity medium having a permittivity the real part of which is negative relative to the electromagnetic wave. A potential-adjusting portion is arranged between the connecting region and the first clad and between the connecting region and the second clad for adjusting the electric potential of the connecting region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a laser device. More specifically, the presentinvention relates to a quantum cascade laser device having repetitivelyarranged active regions. The present invention particularly relates to aquantum cascade laser device adapted to operate in a frequency bandwithin a frequency range extending from the millimeter wave band to theterahertz band (30 GHz to 30 THz).

2. Description of the Related Art

New type semiconductor lasers based on intersubband transitions ofcarriers in the conduction band or in the valence band, hence within asingle energy band, are known and referred to as quantum cascade lasers.Since the oscillation wavelength of a quantum cascade laser depends onthe energy gap of two subbands involved in the optical transition, itcan be selected from a broad spectral range (extending from themid-infrared region to the terahertz band). Japanese Patent ApplicationLaid-Open No. H08-279647 disclosed for the first time that such a lasercan be realized by way of an arrangement of selecting an oscillationwavelength of 4.2 μm in the mid-infrared region.

Recently, longer wavelength lasers have been developed to exploit a longwavelength region relative to the mid-infrared region for theoscillation wavelength in order to meet the demand for electromagneticwave resources of the terahertz band that is believed to be useful forbiosensing applications. Nature, Vol. 417, 156 (2002), describes a laseroscillation at 67 μm (4.5 THz) in the terahertz band. Appl. Phys. Lett.,Vol. 83, 2124 (2003), describes a laser oscillation at a longerwavelength of about 100 μm (3 THz), involving a surface plasmonwaveguide, which includes a negative permittivity medium, the real partof permittivity thereof being negative for the oscillation wavelength.

The configuration of a quantum cascade laser will be summarily describedbelow by referring to FIGS. 4A and 4B of the accompanying drawings,which also illustrate the band profiles employed in the example as willbe described hereinafter.

FIG. 4A illustrates part of the conduction band structure when adesigned electric field is applied to a quantum cascade laser. Referringto FIG. 4A, active region 440 is formed by barriers 441, 443 and 445 andquantum wells 442, 444 and 446. With this arrangement, subbands 411, 412and 413 are formed in the active region 440. Relaxation region 450 isformed by barriers 451, 453, 455 and 457 and quantum wells 452, 454, 456and 458. With this arrangement, a miniband 421 bundling a plurality ofsubbands is formed. Quantum cascade lasers are characterized in thatactive regions and relaxation regions are repetitively and alternatelyarranged. In FIG. 4A, active region 460 is the active region thatappears next in the repetitive arrangement. When a designed electricfield is applied to such a quantum cascade laser, an electric currentoccurs in a manner as described below.

Electrons make an optical transition 401 from the subband 411 to thesubband 412 in the active region 440 to emit light of a wavelength thatcorresponds to the energy gap between the subband 411 and the subband412. Subsequently, the electrons in the subband 412 of the active region440 pass through the subband 413 due to optical phonon scattering 402 soas to be extracted into a relaxation region 450, securing the populationinversion between the subband 411 and the subband 412. Then, theelectrons pass through the miniband 421 in the relaxation region 450 andare injected into the next active region 460 to repeat the same opticaltransition as in the active region 440. The relaxation region 450 isreferred to as “injector” because the relaxation region 450 injectscarriers into the next repeated active region. One or more than one ofthe quantum wells in a relaxation region are slightly doped withcarriers.

SUMMARY OF THE INVENTION

However, known quantum cascade lasers are accompanied by a risk that theapplied voltage may not be uniformly distributed to the active regionsthat are cascade-connected in the laser. In other words, the electricfield may not be uniformly distributed among the active regions asillustrated in FIG. 4B and show a difference from the designed electricfield as illustrated in FIG. 4A. Such a phenomenon is described asformation of a high-field domain in a multiple quantum well structure inthe conduction band or the valence band, hence in a single energy band,in Phys. Rev. B, Vol. 35, 4172 (1987).

It is believed that a high-field domain is apt to be formed when theenergy broadening of subbands is narrow in a multiple quantum wellstructure. Therefore, formation of a high-field domain is a problem tolong wavelength lasers for the reason as described below. Namely, a longwavelength laser requires two subbands showing a narrow energy gap foroptical transitions. In other words, a narrow energy broadening ofsubbands needs to be designed for realizing population inversion. On theother hand, a narrow energy broadening of subbands should not bedesigned for the purpose of suppressing formation of a high-fielddomain. Thus, designing a long wavelength laser is accompanied by aproblem of tradeoff between realization of population inversion andsuppression of formation of a high-field domain.

If a high-field domain is formed, it degrades the laser oscillationcharacteristics. More specifically, if the electric field intensityvaries among active regions, it may no longer be possible to injectcarriers in a designed manner so that the density of populationinversion can fall. Then, the current injection efficiency falls to byturn give rise to a risk of lowering the laser oscillation output andthe drive temperature.

Thus, in the first aspect of the present invention, there is provided alaser device having: a gain medium; a first clad and a second cladsandwiching the gain medium in the thickness direction thereof; and acavity structure for resonating an electromagnetic wave generated in thegain medium, the gain medium having a plurality of active regions forgenerating an electromagnetic wave and at least one connecting regionlaid among the active regions to connect the active regions, the activeregions and the connecting region each including quantum wells andbarriers, the first clad and the second clad each including a negativepermittivity medium, the real part of permittivity thereof beingnegative for the electromagnetic wave, a potential-adjusting portionbeing arranged between the connecting region and the first clad andbetween the connecting region and the second clad, for adjusting theelectric potential of the connecting region.

In the second aspect of the invention, there is provided a laser deviceincluding: a gain medium; a first clad and a second clad sandwiching thegain medium in the thickness direction thereof; and a cavity structurefor resonating the electromagnetic wave generated in the gain medium,the electromagnetic wave having a frequency in a frequency range from 30GHz to 30 THz, the gain medium having a plurality of active regions forgenerating an electromagnetic wave and at least one connecting regionlaid among the active regions to connect the active regions, the activeregions and the connecting region each including quantum wells andbarriers, the active regions being resonant tunneling diodes utilizingthe photon assisted-tunneling phenomenon, the first clad and the secondclad each including a negative permittivity medium, the real part ofpermittivity thereof being negative for the electromagnetic wave, aresistor being arranged between the connecting region and the first cladand between the connecting region and the second clad, for adjusting theelectric potential of the connecting region, the voltages applied to theactive regions arranged adjacent to the connecting region beingequalized by adjusting the electric potential of the connecting region,the laser device being arranged on a substrate, the connecting regionhaving a draw-out layer to be drawn out in the in-plane direction of thesubstrate and electrically connected to the resistor, the draw-out layerbeing carrier-doped and having a finite sheet resistance.

Thus, since a laser device according to the present invention has apotential-adjusting portion, the device can adjust the voltage appliedto each of the active regions connected in a cascade manner. Forexample, the applied voltage can be substantially uniformly distributedamong the active regions to consequently improve the current injectionefficiency of the quantum cascade laser and hence the laser oscillationcharacteristics (including the laser oscillation output and the drivetemperature).

According to the present invention, a potential-adjusting portion forsuppressing formation of a high-field domain is structurally arranged asa separate part from its multiple quantum well structure. Then, theenergy broadening of subbands can be reduced without the problem oftradeoff that arises when designing a long wavelength laser. Therefore,quantum cascade lasers that can operate at a still longer wavelengthside can be designed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of an embodiment of laserdevice according to the present invention, illustrating theconfiguration thereof, and FIG. 1B is an equivalent electric circuitdiagram of the embodiment of FIG. 1A.

FIG. 2 is a schematic cross sectional view of the laser device ofExample 1, illustrating the configuration thereof.

FIG. 3A is a schematic cross sectional view of a laser device realizedby modifying the laser device of Example 1, and FIG. 3B is an enlargedview of the gain medium of the laser device of FIG. 3A.

FIG. 4A is a diagram illustrating the band profile of the conductionband structure of the laser device of Example 1, and FIG. 4B is adiagram illustrating an exemplary band profile of the conduction bandstructure of a known laser device.

FIG. 5 is a schematic cross sectional view of the laser device ofExample 2, illustrating the configuration thereof.

FIG. 6 is a diagram illustrating the band profile of the conduction bandstructure of the laser device of Example 2.

FIG. 7A is a schematic cross sectional view of the laser device ofExample 3, and FIG. 7B is an enlarged view of the gain medium of thelaser device of FIG. 7A.

FIG. 8A is a schematic cross sectional view of the laser device ofExample 4, and FIG. 8B is an enlarged view of the gain medium of thelaser device of FIG. 8A.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in greater detail byreferring to the accompanying drawings that illustrate variousembodiments of the present invention. FIG. 1A is a schematic crosssectional view of an embodiment of laser device according to the presentinvention taken perpendicularly relative to the direction of propagationof electromagnetic wave, illustrating the configuration thereof.

The laser device of this embodiment has a gain medium 103, a first cladand a second clad sandwiching the gain medium in the thickness directionthereof and a cavity structure for resonating electromagnetic wavesgenerated in the gain medium. The gain medium by turn has a plurality of(two in this case) active regions 140 and 160 and at least one (one inthis case) connecting region 150 interposed among (between) the activeregions so as to connect the active regions. Note that not a singleconnecting region 150 but a plurality of connecting regions may bearranged in a laser device. For the purpose of the present invention,the concept of “connecting region” includes the concept of “relaxationregion” mentioned above by referring to FIG. 4A. More specifically, aconnecting region is adapted to inject carriers into an active region,as a relaxation region. However, for the purpose of cascade connectionof the present invention, a connecting region may inject carriers intoan active region by way of an energy relaxation process or not by way ofan energy relaxation process. An active region and a connecting regionboth include quantum wells and barriers.

As will be described in greater detail hereinafter, both the first cladand the second clad are formed so as to include a negative permittivitymedium, the real part of permittivity thereof being negative forelectromagnetic waves generated in the gain medium. Thus, a waveguide isformed by the first clad and the second clad for the gain medium. Thepresent invention is mainly characterized by the feature that apotential-adjusting portion is arranged between the connecting regionand the first clad and between the connecting region and the secondclad. With such an arrangement, by adjusting the electric potential ofthe connecting region, the voltages applied respectively to the activeregions that are adjacently located at the opposite sides of theconnecting region can be adjusted and equalized.

Now, the laser device of this embodiment and the characteristics thereofwill be described in detail below.

The laser device has a gain medium 103 that extends along the directionof propagation of electromagnetic waves. The gain medium 103 includes acouple of active regions 140 and 160 that emit an electromagnetic wave,utilizing optical transitions. The gain medium 103 additionally has aconnecting region 150. As pointed out above, the connecting regiontypically gives rise to a rapid energy relaxation to carriers comingfrom the preceding active region 140 and injects carriers into thesucceeding active region 160. In the gain medium 103, the connectingregion 150 shows an arrangement similar to that of a relaxation regionof a known quantum cascade laser. The active regions 140 and 160 and theconnecting region 150 have a net gain as a whole when a currentinjection takes place.

The gain medium 103 is sandwiched by a pair of negative permittivitymediums 101 and 102 in the thickness direction thereof. An electriccontact needs to be established between the gain medium 103 and thenegative permittivity mediums 101 and 102 when externally injecting anelectric current. Contact layers 111 and 112 are provided for thispurpose. The first (lower) clad is formed by the negative permittivitymedium 101 and the contact layer 111, while the second (upper) clad isformed by the negative permittivity medium 102 and the contact layer112.

A negative permittivity medium is a substance having a negative realpart of permittivity in the frequency range of the electromagnetic waveto be oscillated. The negative permittivity medium is formed by acarrier-doped semiconductor, a metal or a plurality of substancesincluding these substances (a metal and a carrier-doped semiconductor)in a frequency range extending from the millimeter wave band to theterahertz band. A transparent electroconductive film may be selectedbecause a negative permittivity medium is typically an electroconductivematerial.

Since the gain medium 103 (the active regions 140 and 160 and theconnecting region 150) has a semiconductor multilayer structure, acarrier-doped semiconductor is selected for the contact layers 111 and112. As such a substance is selected for the contact layers 111 and 112,the negative permittivity mediums 101 and 102 and the contact layers 111and 112 operate as a pair of clads having a geometrical opticalreflecting surface. Surface plasmon having no diffraction limit isallowed as guided mode for the electromagnetic wave that is guided bythe first clad (the negative permittivity medium 101 and the contactlayer 111) and the second clad (the negative permittivity medium 102 andthe contact layer 112).

Electrodes 121 and 122 are formed respectively on the negativepermittivity mediums 101 and 102 and connected to an external fieldapplication section (not illustrated). In this embodiment, theconnecting region 150 further has a draw-out layer drawn out in thetransversal direction, which is the in-plane direction of the connectingregion. The draw-out layer is held in contact with electrodes 159.Electric resistances 12A and 12B are electrically connected respectivelybetween the electrode 121 and the electrode 159 and between theelectrode 122 and the electrode 159 by means of a resistor 12. Theyoperate as structure for substantially equally dividing the voltageapplied by external field application to the active regions 140 and 160.In other words, the structure operates as a potential-adjusting portionarranged between the connecting region 150 and the first clad and alsobetween the connecting region 150 and the second clad so as to adjustthe electric potential of the connecting region 150. In FIG. 1A, numeral11 denotes a transfer substrate.

FIG. 1B is an equivalent electric circuit diagram of the above describedlaser device. The voltage at the first clad (the negative permittivitymedium 101 and the contact layer 111) having an electric potentialsubstantially equal to the electric potential of the electrode 121 isdenoted by V101 in FIG. 1B. Similarly, the voltage at the second clad(the negative permittivity medium 102 and the contact layer 112) havingan electric potential substantially equal to the electric potential ofthe electrode 122 is denoted by V102 in FIG. 1B. The active regions 140and 160 operate as so many loads and the load resistances arerespectively denoted by R140 and R160 in FIG. 1B. The electricresistances 12A and 12B of the resistor 12 are respectively denoted byR12A and R12B in FIG. 1B.

The connecting region 150 is slightly carrier-doped. Therefore, theconnecting region 150 has a limited sheet resistance, which is denotedby R150 in FIG. 1B. In this embodiment, the resistor 12 operates in amanner as described below by way of the limited sheet resistance.Generally, the electric potential of the connecting region 150 varies asa function of the spot therein due to the sheet resistance R150. In FIG.1B, the voltage of the part of the connecting region 150 sandwiched bythe active regions 140 and 160 is denoted by V150 and the voltage of thepart of the connecting region 150 located near the electrode 159 isdenoted by V150′. As is known in the field of analog circuittechnologies, the equation of V150=V150′ holds true for the voltages ofthe connecting region 150 when formula 1 shown below holds true.

R140×R12B=R160×R12A  (formula 1)

While the above equation is cited as an example, it indicates that thevalue of V150 can be controlled by changing the electric resistances 12Aand 12B of the resistor 12.

When the resistor 12 satisfies the requirement of the formula 1 and theelectric resistances 12A and 12B are so low as to make the loadresistances negligible and expressed by formula 2 shown below, the valueof V150 can be controlled by controlling the electric potential of theresistor 12.

R12A<R140, R12B<R160  (formula 2)

Additionally, when the electric resistances 12A and 12B are equal toeach other, or R12A=R12B, V150 is substantially at the middle betweenV102 and V101. Therefore, the voltage applied by external fieldapplication is divided substantially equally into the load resistanceR140 and the load resistance R160. Thus, the resistor 12 canstructurally suppress formation of a high-field domain in the gainmedium 103.

The gain medium 103 of FIG. 1A so appears that it is formed byrepetitively arranging two active regions. However, the gain medium 103can be formed by repetitively arranging three or four active regions.Generally, the gain medium 103 can be formed by repetitively arranging aplurality of active regions. When the number of the repetitivelyarranged active regions is large, only the electric potentials of someof the connecting regions connected to the related active regions may becontrolled. Generally, a potential-adjusting portion is arranged for atleast a connecting region between the connecting region and the firstclad and between the connecting region and the second clad.

The unit of repetition of an active region and a connecting region maybe of the popular bound-to-continuum type, of the 4-well type or of the3-well type. A resonant tunneling diode may be employed for each of theactive regions. Then, a doped semiconductor layer having an appropriatethickness may be employed for the connecting region. A resonanttunneling diode that utilizes a photon-assisted tunneling phenomenon maybe employed for each of the active regions. Any of them generates anelectromagnetic wave gain in the frequency range of the millimeter waveband or the terahertz band when an electric current is injected.

While the resistor 12 is only abstractedly expressed above in the abovedescription of the embodiment, a film resistor made of anelectroconductive material may specifically be employed for the resistor12, although the present invention is by no means limited thereto and achip resistor can be a means operating for the potential-adjustingportion. Additionally, a chip inductor or a chip bead can also beemployed as means for operating as the potential-adjusting portionbecause external field application is not limited to a DC. In otherwords, the potential-adjusting portion is only required to be arrangedso as to connect a cascade connecting region and the related first cladand the cascade connecting region and the related second clad by DCconnection or by AC connection. However, the use of a film resistor ispreferable from the viewpoint of monolithically realizing the resistor12. Note that an electroconductive material that can realize arelatively low resistance value is preferably used for such a filmresistor. Examples of such materials include semimetals (e.g., Bi,graphite) and transparent electroconductive films (e.g., ITO, ZnO andZnSn). A semiconductor showing a relatively high electric conductivitysuch as poly-Si may also be used for such a film resistor. The filmresistor 12 may be continuously arranged along the direction ofpropagation of surface plasmon (the direction perpendicular to FIG. 1A)but alternatively it may be arranged in the form of a lumped constantelement that is formed in an undistributed manner. Preferably, the lossfor surface plasmon can be minimized by selecting nodes of theelectromagnetic wave to be oscillated and arranging them in the form ofa lumped constant element.

An external field control section having a sufficient drive capacity maywell be utilized for driving such a laser device by taking the currentinjection to the gain medium 103 and the electric current flowingthrough the resistor 12 for suppressing formation of a high-field domaininto consideration.

Thus, this embodiment can substantially uniformly distribute the voltageamong the active regions cascade-connected with connecting regionsinterposed among them due to the above described potential-adjustingportion it has. The laser oscillation characteristics can be improved.

EXAMPLE

The present invention will be described further by way of examplesparticularly in terms of specific configurations.

Example 1

FIG. 2 is a schematic cross sectional view of the laser device ofExample 1 taken perpendicularly relative to the direction of propagationof electromagnetic wave. In this Example, the gain medium 203 has aconfiguration of repetitively arranging two active regions 240 and 260that can be used for a quantum cascade laser. Thus, there is a singlecascade connecting region 250 arranged between the active regions 240and 260. The profile of the bands is designed as a bound-to-continuumtype multiple quantum well structure as illustrated in FIG. 4A. As forthese regions, a lattice-matched GaAs well layer and a lattice-matchedor lattice-unmatched AlGaAs barrier layer may be formed on a GaAssubstrate.

More specifically, a semiconductor multilayer film structure as shownbelow may be arranged from the emitter side to the collector side as aunit of repetition (cited from Nature, Vol. 417, 156 (2002)). AlGaAs 4.3nm (441)/GaAs 18.8 nm (442)/AlGaAs 0.8 nm (443)/GaAs 15.8 nm(444)/AlGaAs 0.6 nm (445)/GaAs 11.7 nm (446)/AlGaAs 2.5 nm (451)/GaAs10.3 nm (452)/AlGaAs 2.9 nm (453)/GaAs 10.2 nm (454)/AlGaAs 3.0 nm(455)/GaAs 10.8 nm (456)/AlGaAs 3.3 nm (457)/GaAs 9.9 nm (458).

Of the above, from the top layer to the 2.5 nm-thick AlGaAs 451 layerare active regions 240 and 260 and from the 2.5 nm-thick AlGaAs 451layer to the bottom layer constitute a connecting region 250. The 10.2nm-thick n-GaAs 454 layer is a carrier-doped injector layer that shows aslight electron density of about 4×10¹⁶ cm⁻³. The other layers areundoped layers which are not intentionally doped.

In this example, the 10.2 nm-thick n-GaAs layer 454 in the cascadeconnecting region 250 is a draw-out layer. The carriers (electrons arechosen here) injected from the emitter produce an electric currentshowing a current density of about 1 kA/cm² when an electric field ofabout 3.5 kV/cm is uniformly applied to the above semiconductormultilayer film structure to produce a gain in a frequency range of theterahertz band due to intersubband transitions. The gain medium 203 issandwiched between negative permittivity mediums 201 and 202 that alsooperate as contact layers. The contact layers 201 and 202 of the gainmedium 203 are typically formed by means of a semiconductor film ofn-GaAs (200 nm thick) that is lattice-matched to the GaAs substrate. Anelectron density of 5×10¹⁸ cm⁻³ is selected here. The gain medium 203and the negative permittivity mediums 201 and 202 are formed on the GaAssubstrate typically by means of a semiconductor epitaxial growth. FIG. 2illustrates the structure produced after transferring these epitaxiallayers onto a transfer substrate 21. The GaAs substrate is alreadyremoved and the negative permittivity mediums 201 and 202 are held incontact with respective electrodes 221 and 222 typically made of Ti/Au.

However, it should be noted that only an example of structure that canbe formed on a GaAs substrate is described above and the presentinvention is by no means limited thereto. A semiconductor multilayerstructure of arranging InAs/AlAsSb on an InAs substrate, arrangingInGaAs/InAlAs, InGaAs/AlAs or InGaAs/AlGaAsSb on an InP substrate orarranging Si/SiGe on an Si substrate is also feasible. When Si/SiGearranged on an Si substrate is utilized, holes may be used as carriers.

The surface plasmon waveguide as described above is typically made toshow a length of 1,000 μm in the direction of propagation of surfaceplasmon and a width of 20 μm in a direction transversal to the abovedirection. The draw-out layer 454 is drawn out in the transversaldirection by 50 μm. Then, the load resistance of the active regions 240and 260 is of the order of 0.1Ω. If the above formula 2 is madeapplicable to the resistor 22 for the purpose of simplicity, a resistorshowing a resistance lower than the load resistance may well be used forthe resistor 22. A ribbon-shaped resistor arranged to cross the surfaceplasmon waveguide as illustrated in FIG. 2 is adopted here. When a Bifilm was formed by evaporation, the film showed a resistivity of about 3Ω·μm. Therefore, when the ribbon-shaped Bi film resistor 22 is made toshow a thickness of 1 μm and a width (the length in the direction ofpropagation of surface plasmon) of 3 μm or more, the following resultsare obtained. Namely, both the resistance value between the electrode221 and the electrode 259 on the draw-out layer 454 and the resistancevalue between the electrode 222 and the electrode 259 are equal to eachother and of the order of 0.01Ω or less so that the requirement of theformula 2 is satisfied. The passivation film 23 for protecting thelateral wall of the laser device has a film thickness of about 200 nm.Thus, the Bi film resistor 22 structurally suppresses formation of ahigh-field domain in the gain medium 203 as described above by referringto the embodiment. The passivation film 23 and the electrode 259 maytypically be made of SiO₂ and Ti/Au respectively.

The laser device of this example can be prepared in a manner asdescribed below. Firstly, an etch-stop layer of AlGaAs is formed bymolecular beam epitaxy (MBE) and an n-GaAs layer 202, a GaAs/AlGaAsmultiple quantum well layer 203 and an n-InGaAs layer 201 are formed ona GaAs substrate by epitaxial growth. Then, after forming an electrode221 of Ti/Au on the surface thereof by evaporation, the GaAs substrateis polished to a thickness of about 120 μm. Thereafter, a 1,000 μm cubicchip is formed from the work by cleavage and the electrode 221 and theAu thin film formed on a transfer substrate 21, on which a Ti/Au thinfilm is formed by evaporation, are bonded to each other by way of apressure bonding process. Alternatively, they may be bonded to eachother by way of a heat bonding process, using solder of AuSn forexample. Only the GaAs substrate is selectively etched out up to theetch-stop layer when the work is subjected to a wet etching process,using ammonia water and aqueous hydrogen peroxide. Then, a mesa-shapedepitaxial layer is transferred onto the transfer substrate 21.

Subsequently, a 50 μm-wide mesa shape is produced by photolithographyand dry etching until the underlying layer is exposed. Thereafter, a 20μm-wide mesa shape is produced by etching to expose the draw-out layer454, following a similar process. A Degilem may be used to measure theetching depth in-situ for the purpose of realizing a highly accurateetching process. A selective wet etching process may be adopted here.

As a result of the above-described process, a stripe-shaped waveguidehaving a cavity length of 1,000 μm and cleavage planes arranged at theopposite ends is formed as cavity structure. Subsequently, a SiO₂ film23 is formed typically by plasma CDV and then the mesa-shaped epitaxiallayer is exposed except a lateral wall.

The lateral wall can be left by etching, opening a stripe-shaped windowhaving a width of 50 μm or less by patterning. Additionally, a Ti/Auelectrode 222 and another Ti/Au electrode 259 are formed respectively onthe surface of the n-GaAs layer 202 and on the surface of the draw-outlayer 454 in the connecting region 250 by way of a lift-off process.Finally, a Bi film resistor 22 is formed by way of a lift-off process toproduce a complete device. An electric current flows as described aboveby referring to FIG. 4A when a predetermined electric field is appliedto the quantum cascade laser.

Alternatively, three or more than three active regions may berepetitively arranged. FIG. 3A is a schematic cross sectional view of alaser device realized by modifying the laser device of Example 1. Atotal of six active regions 340 are repetitively arranged there. FIG. 3Bis an enlarged view of the gain medium 303 of the laser device of FIG.3A. Thus, the device has five connecting regions 350. In this modifiedexample, not all but two connecting regions 350 are drawn out and heldin contact with a resistor 32 in order to reduce the number ofmanufacturing steps. With this arrangement, formation of a high-fielddomain in the gain medium 303 can be suppressed if compared with anarrangement where no resistor 32 is added.

The ratio of the resistance value of the resistor 32 arranged betweenthe (upper) electrode 359 on the connecting region 350 and the electrode322 on the clad (negative permittivity medium 302) to the resistancevalue of the resistor 32 arranged between the electrode 359 and theelectrode on the clad (negative permittivity medium 301) is determinedin a manner as described below. While there are two active regions 340between the connecting region 350 and the negative permittivity medium302, there are four active regions 340 between the connecting region 350and the negative permittivity medium 301. Thus, the ratio is equal to1:2. Similarly, the ratio of the resistance values of the two resistors32 is determined in a manner as described below. The ratio of theresistance value of the resistor 32 between the (lower) electrode 359 onthe connecting region 350 and the electrode 322 on the clad (negativepermittivity medium 302) to the resistance value of the resistor 32between the electrode 359 and the clad (negative permittivity medium301) is determined substantially equal to 2:1. In this way, the voltageapplied by external field application is substantially equally dividedto the pairs of active regions 340 arranged side by side with aconnecting region 350 interposed between them to produce a structurethat can suppress formation of a high-field domain in the gain medium303. In FIG. 3A, numeral 31 denotes a transfer substrate and numeral 33denotes a passivation film for protecting the lateral wall, while 321denotes an electrode.

As for the ratio of the resistance values, if there are m active regionsbetween the connecting region and the first clad and n active regionsbetween the connecting region and the second clad, the ratio of theresistance value of the resistor arranged between the former componentsto the resistance value of the resistor arranged between the lattercomponents is determined to be substantially equal to m:n.

A transparent electroconductive film showing a resistivity relativelyclose to that of a semimetal film resistor such as a Bi film resistormay alternatively be used for the purpose of the present invention. AnITO film or a ZnO film having a profile similar to the above-describedone may be employed in this example.

Thus, the laser device of this example can substantially uniformlydistribute the voltage applied thereto to the cascade-connected activeregions with connecting regions interposed among them to improve thecurrent injection efficiency and hence the laser oscillationcharacteristics of the quantum cascade laser.

Example 2

FIG. 5 is a schematic cross sectional view of the laser device ofExample 2 taken perpendicularly relative to the direction of propagationof electromagnetic wave. In this Example, the gain medium 503 has aconfiguration of repetitively arranging two active regions 540 and 560that are formed by using three barrier layers for each and can be usedfor a resonant tunneling diode. Thus, there is a single connectingregion 550 arranged between the active regions 540 and 560. The profileof the bands is designed as a multiple quantum well structure asillustrated in FIG. 6. As for these regions, a lattice-matched InGaAswell layer and a lattice-matched InAlAs layer or a lattice-unmatchedAlAs barrier layer may be formed on an InP substrate.

More specifically, a semiconductor multilayer film structure as shownbelow may be arranged from the emitter side to the collector side as aunit of repetition.

AlAs 1.3 nm (641)/InGaAs 7.6 nm (642)/InAlAs 2.6 nm (643)/InGaAs 5.6 nm(644)/AlAs 1.3 nm (641)/InGaAs 5.4 nm (652)/InAlAs 0.6 nm (653)/InGaAs5.4 nm (654)/InAlAs 0.6 nm (653)/InGaAs 5.4 nm (654)/InAlAs 0.6 nm(653)/InGaAs 5.4 nm (654)/InAlAs 0.6 nm (653)/InGaAs 5.4 nm (656)

Of the above, from the top layer to the 1.3 nm-thick AlAs 651 layer areactive regions 540 and 560 and from the 1.3 nm-thick AlAs 651 layer tothe bottom layer constitute a connecting region 550. The 5.4 nm-thickn-InGaAs 654 layer is a carrier-doped injector layer that employs aminiband 621 for cascade connection and shows a relatively high electrondensity of about 2×10¹⁸ cm⁻³. The other layers are undoped layers whichare not intentionally doped.

In this example, the 5.4 nm-thick n-InGaAs layer 654 in the connectingregion 550 is a draw-out layer. The carriers (electrons are chosen here)injected from the emitter produce an electric current showing a currentdensity of about 90 kA/cm² when an electric field of about 40 kV/cm isuniformly applied to the above semiconductor multilayer film structureto produce a gain in a frequency range of the terahertz band due tophoton-assisted tunneling. (Refer to Jpn. J. Appl. Phys., Vol. 40, 5251(2001) for the relationship between photon-assisted tunneling and gain).

The contact layers 511 and 512 of the gain medium 503 are typicallyformed by means of a semiconductor film of n-InGaAs (50 nm thick) thatis lattice-matched to the InP substrate. Electrons are chosen ascarriers and Si is employed as dopant. An electron density of 2×10¹⁸cm⁻³ is selected here. The gain medium 503 is sandwiched betweennegative permittivity mediums 501 and 502, which are also formed bymeans of an n-InGaAs (100 nm thick) semiconductor film that islattice-matched to the InP substrate and made to show an electrondensity of 1×10¹⁹ cm⁻³. The gain medium 503, the negative permittivitymediums 501 and 502 and the contact layers 511 and 512 are formed on theInP substrate typically by means of a semiconductor epitaxial growth.FIG. 5 illustrates the structure produced after transferring theseepitaxial layers onto a transfer substrate 51. The InP substrate isalready removed and the negative permittivity mediums 501 and 502 areheld in contact with respective electrodes 521 and 522 typically made ofTi/Pd/Au.

However, it should be noted that only an example of structure that canbe formed on an InP substrate is described above and the presentinvention is by no means limited thereto. A semiconductor multilayerstructure of arranging InAs/AlAsSb or InAs/AlSb on an InAs substrate,arranging GaAs/AlGaAs on a GaAs substrate, arranging InGaAs/AlGaAsSb onan InP substrate or arranging Si/SiGe on an Si substrate is alsofeasible.

FIG. 6 illustrates how an electric current flows when a predeterminedelectric field is applied to such a cascade-connected resonant tunnelingdiode (gain medium 503). Referring to FIG. 6, electrons make aphoton-assisted tunneling transition 601 from a subband 611 to anothersubband 612 in the active region 640 to emit light of a wavelength thatcorresponds to the energy gap between the subband 611 and the subband612. Subsequently, the electrons in the subband 612 of the active region640 are extracted into a connecting region 650. Then, the electrons thatpass through the connecting region 650 are injected into the next activeregion 660 to repeat the same photon-assisted tunneling transition as itmade in the active region 640. The population inversion of electrons issecured between the subband 611 and the subband 612 according to thequasi-Fermi distribution for electrons in the miniband 621 of theconnecting region 650.

The surface plasmon waveguide as described above is typically made toshow a length of 300 μm in the direction of propagation of surfaceplasmon and a width of 5 μm in a direction transversal to the abovedirection. The draw-out layer 654 is drawn out in the transversaldirection by 20 μm. Then, the load resistance of the active regions 540and 560 is of the order of 1Ω. A resistor showing a resistance lowerthan the load resistance may well be used for the resistor 52 as inExample 1. A ribbon-shaped Bi resistor arranged to cross the surfaceplasmon waveguide as illustrated in FIG. 5 is adopted here. Therefore,when the ribbon-shaped Bi film resistor 52 is made to show a thicknessof 0.3 μm and a width (the length in the direction of propagation ofsurface plasmon) of 3 μm or more, the following results are obtained.Namely, both the resistance value between the electrode 511 and theelectrode 559 and the resistance value between the electrode 522 and theelectrode 559 are equal to each other and of the order of 0.1Ω or lessso that the requirement of the formula 2 is satisfied. The passivationfilm 53 for protecting the lateral wall of the laser device has a filmthickness of about 200 nm as in Example 1. Thus, the Bi film resistor 52structurally suppresses formation of a high-field domain in the gainmedium 503 as described above by referring to the embodiment. Thepassivation film 53 and the electrode 559 may typically be made of SiO₂and Ti/Pd/Au respectively.

The laser device of this example can be prepared in a manner asdescribed below. Firstly, n-InGaAs layers 502 and 512, an InGaAs/AlAs orInGaAs/InAlAs multiple quantum well layer 503 and n-InGaAs layers 511and 501 are formed on an InP substrate by epitaxial growth using MBE orthe like. Then, after forming an electrode 521 of Ti/Pd/Au byevaporation on the surface thereof, the InP substrate is polished to athickness of about 120 μm. Thereafter, a 300 μm cubic chip is formedfrom the work by cleavage and then bonded to the Au thin film on atransfer substrate 51, on which the electrode 521 and the Ti/Au thinfilm are formed by evaporation, by way of a pressure bonding process.Alternatively, they may be bonded to each other by way of a heat bondingprocess, using solder of AuSn for example. Only the InP substrate isselectively etched out when the work is subjected to a wet etchingprocess, using hydrochloric acid. Then, a mesa-shaped epitaxial layer istransferred onto the transfer substrate 51.

Subsequently, a 20 μm-wide mesa shape is produced by photolithographyand dry etching until the underlying layer is exposed. Thereafter, a 5μm-wide mesa shape is produced by etching to expose the draw-out layer654, following a similar process. A Degilem may be used to measure theetching depth in-situ for the purpose of realizing a highly accurateetching process. A selective wet etching process may be adopted here.

As a result of the above-described process, a stripe-shaped waveguidehaving a cavity length of 300 μm and cleavage planes arranged at theopposite ends is formed. Subsequently, a SiO₂ film 53 is formedtypically by plasma CVD and then the mesa-shaped epitaxial layer isexposed except the lateral wall. A lateral wall can be left by etching,opening a stripe-shaped window having a width of 20 μm or less bypatterning. Additionally, a Ti/Pd/Au electrode 522 and another Ti/Pd/Auelectrode 559 are formed respectively on the surface of the n-InGaAslayer 502 and on the surface of the draw-out layer 654 in the cascadeconnecting region 550 by way of a lift-off process. Finally, a Bi filmresistor 52 is formed by way of evaporation to produce a completedevice.

Alternatively, three or more than three active regions may berepetitively arranged as in Example 1. A resonant tunneling diode formedby using two barrier layers may be used for active regions instead of aresonant tunneling diode formed by using three barrier layers. Witheither arrangement, the resistor 52 suppresses formation of a high-fielddomain because carriers pass through multiple quantum well structure ofthe conduction band or the valence band, hence within a single band.

Thus, the laser device of this example can substantially uniformlydistribute the voltage applied thereto to the cascade-connected activeregions with connecting regions interposed among them to improve thecurrent injection efficiency and hence the laser oscillationcharacteristics of the quantum cascade laser.

Example 3

FIG. 7A is a schematic cross sectional view of the laser device ofExample 3, which is formed by modifying the laser device of Example 1,taken perpendicularly relative to the direction of propagation ofelectromagnetic wave, illustrating the configuration thereof. The gainmedium 703 of this example is formed by repetitively arranging gainmediums 203 of Example 1 for n times (three times in the illustratedarrangement) (see FIG. 7B). The output power level of a quantum cascadelaser is raised when the number of times of repetition is increased.Therefore, the device of this example is advantageous from such a pointof view. Otherwise, the device of this example is the same as that ofExample 1.

While the device of this example has a configuration substantially thesame as the device of Example 1, the lateral wall of the device isforwardly tapered as illustrated in FIG. 7A because the resistor isarranged to accommodate the repetitive arrangement. At the same time,the resistor 72 is held in direct contact with the connecting regions750. For this reason, poly-Si may advantageously be used for theresistor 72. The poly-Si film resistor 72 of this example can be formedby plasma CVD. The device can suppress formation of a high-field domainin the gain medium 703 when a ribbon-shaped resistor is formed so as tocross the surface plasmon waveguide as in Example 1. The resistancevalues of the resistor 72 arranged between adjacent pairs of cascadeconnecting regions 750 are made substantially equal to each other so asto equally divide the voltage applied to the device by external fieldapplication for the active regions 740.

The device of this example is characterized by the forwardly taperedprofile thereof. Such a profile can be produced by defining a waveguidein the forward direction of the mesa stripe and conducting a wet etchingprocess using a mixed aqueous solution of sulfuric acid and hydrogenperoxide or alternatively a dry etching process, forming a taperedresist layer. Any known appropriate techniques can be used to form apassivation film 73 for protecting the lateral wall, a resistor 72 andelectrodes 721 and 722. In FIG. 7A, numeral 71 denotes a transfersubstrate and numerals 701 and 702 denote respectively negativepermittivity mediums, while numerals 711 and 712 denote respectivelycontact layers.

Example 4

FIG. 8A is a schematic cross sectional view of the laser device ofExample 4, which is formed by modifying the laser device of Example 3,taken perpendicularly relative to the direction of propagation ofelectromagnetic wave, illustrating the configuration thereof. The gainmedium 803 of this example is formed by repetitively arranging gainmediums 203 of Example 1 for n times (see FIG. 8B). Additionally, thetransverse mode of surface plasmon is shaped to reduce the waveguideloss in this example. Therefore, the device of this example isadvantageous from such a point of view. Otherwise, the device of thisexample is the same as that of Example 3.

While the device of this example is formed substantially in the samemanner as in Example 1, the lateral wall is made to show a constrictionas illustrated in FIG. 8A in order to form the resistor 82, whileshaping the transverse mode of surface plasmon. As the same time, theresistor 82 is held in direct contact with the cascade connectingregions 850. For this reason, poly-Si may advantageously be used for theresistor 82. The poly-Si film resistor 82 of this example can be formedby plasma CVD. The device can suppress formation of a high-field domainin the gain medium 803 when a ribbon-shaped resistor is formed so as tocross the surface plasmon waveguide as in Example 1.

The device of this example is characterized by its constricted profile.To produce such a profile, the mesa at the side of the negativepermittivity medium 801 is prepared in advance and the bonding operationis conducted at the top of the mesa. Otherwise, the process of preparinga device of Example 1 may be followed. The lateral wall can also beformed at the constricted part of the mesa when CVD is employed forforming the resistor 82. In FIG. 8A, numerals 81 and 83 respectivelydenote a transfer substrate and a passivation film and numerals 801 and802 denote respectively negative permittivity mediums, while numerals811 and 812 denote respectively contact layers and numerals 821 and 822denote respectively electrodes. In FIG. 8B, 840 denotes an activeregion.

In the above-described examples, the surface plasmon waveguide thatprovides a cavity structure may be of the Fabry-Perot type having endfacets in the direction of propagation of electromagnetic wave. However,as is known in the field of semiconductor lasers, the DFB type ofmodulating a stripe in the direction of propagation or a DBR type ofdistributing reflecting surfaces in the direction of propagation mayalso be feasible for the purpose of the present invention. Other thanplanar end facets may be employed for termination of the surface plasmonwaveguide. For example, a λ/4 impedance transformer may be provided toreduce the mismatching relative to the external space as is known in thefield of microwave technologies. To do this, the waveguide may betapered for λ/4 from each terminal. Alternatively, as is known in thefield of optical technologies, provision of an AR coat formed for λ/4from each terminal may also be feasible.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2007-331275, filed on Dec. 25, 2007, which is hereby incorporated byreference herein in its entirety.

1-10. (canceled)
 11. A device for oscillating an electromagnetic wave,comprising: a gain medium; and a first clad and a second cladsandwiching the gain medium, wherein the gain medium includes: activeregions each having a quantum well structure for generating anelectromagnetic wave, and a connecting region positioned between theactive regions to connect the active regions, wherein the connectingregion and the first clad or the second clad are electrically connectedvia a voltage-adjusting portion for adjusting a voltage to be appliedbetween the connecting region and the first clad or the second clad, andnot via any of the active regions, and wherein the active regionsgenerate electromagnetic waves having substantially a same frequency.12. The device according to claim 11, wherein the active regions have asame multilayer structure.
 13. The device according to claim 12, whereinthe active regions have different areas from each other.
 14. The deviceaccording to claim 11, wherein each of the first clad and the secondclad includes a negative permittivity medium, a real part of apermittivity thereof being negative relative to the electromagneticwaves.
 15. The device according to claim 11, wherein thevoltage-adjusting portion is a resistor.
 16. The device according toclaim 15, wherein the device is arranged on a substrate and theconnecting region includes a draw-out layer drawn out in an in-planedirection of the substrate and electrically connected to the resistor.17. The device according to claim 16, wherein the draw-out layer iscarrier-doped and has a finite sheet resistance.
 18. The deviceaccording to claim 15, wherein the resistor is a film resistor formed ofan electroconductive material.
 19. The device according to claim 15,wherein the resistor is formed of a semimetal, a transparentelectroconductive film, or a semiconductor.
 20. The device according toclaim 14, wherein the negative permittivity medium is formed by a metal,a carrier-doped semiconductor, or a metal and a carrier-dopedsemiconductor.
 21. The device according to claim 11, wherein the activeregions include a resonant tunneling diode that exhibits aphoton-assisted tunneling phenomenon.
 22. The device according to claim11, wherein the electromagnetic waves have a frequency within a rangeextending from 30 GHz to 30 THz.